For proper function of the mature nervous system, neurons send processes out during development that must choose their correct synaptic partners from an enormous population of cells. The importance of this process in the mammalian brain is underscored by the growing knowledge that many neurodevelopmental disorders, such as autism and epilepsy, as well as adult disorders like bipolar disorder and schizophrenia, may result from improper formation of neuronal circuits. Therefore, a full understanding of the molecules mediating synaptic partner choice is crucial for make advancements in our comprehension and treatment of these diseases. While many guidance cues have been identified that assist outgrowing axons to reach an area of the brain, it's less clear how an axon chooses its exact synaptic partner. The clustered protocadherin family of genes (Pcdhs) show great potential to fill this role in mammalian brain development. The Pcdh gene family consists of 58 genes in three gene clusters (1, 2, and 3), each encoding for a unique adhesive transmembrane protein. It has been shown that individual neurons express a wide diversity of Pcdh isoforms, and studies have suggested their importance for synapse formation and axon targeting in certain classes of neurons. Furthermore, linkage analyses of schizophrenia and bipolar disorder patients have uncovered susceptibility loci for both disorders in the human genome near the clustered protocadherin gene family. Yet progress in understanding the function of Pcdhs has been limited by lethality in Pcdh-3 knockout animals, while studies of the other clusters are few (for Pcdh-1) or non-existent (for Pcdh-2). However, by using Mosaic Analysis with Double Markers (MADM), it should be possible to gain novel insight into the mechanisms by which Pcdhs contribute to neural development. Briefly, MADM allows for simultaneous labeling and gene knockout in individual cells. By inserting the appropriate labeling cassettes into chromosome 18 and breeding these mice (called MADM18 mice) with various Pcdh heterozygous mutant mice (1, 2, and 3 mutants, or a mutant mouse where all three clusters have been deleted), it will be possible to analyze axonal projections, dendritic arborizations, and synapse formation of Pcdh-lacking neurons next to wildtype-labeled neurons in a variety of brain regions where these proteins are known to be expressed. Compared to previous Pcdh-knockout studies, this strategy has several benefits: it avoids the lethality associated with complete loss of Pcdh-3, and should provide a more accurate assessment of Pcdh function through single cell knockout of the genes. Characterizing the role of the clustered protocadherins in neuronal development may also have value for understanding the molecular mechanisms of several neurological diseases, such as autism and schizophrenia, where it's thought that improper synapse formation may be involved. Finally, the MADM mouse produced for this study could be used in the future to analyze any gene distal to the MADM cassettes on the chromosome 18, which may be a useful tool for researchers in a variety of scientific fields.